Alpha output without background

Support Compositing and Post Processing
1

Hi everyone,

I was wondering if there’s a way to output pure alpha channel renders to a PNG image in a way that ignores the colour of the background. I’m trying to make some animated sprites of explosions, and I’m rendering against a black background, so when the explosion becomes thin around the edges, the output colour is mixed with the background.As a result, I get an ugly, smudgy black line around the edge of the sprite when I put it against, say, a white background. Any way to solve this?

2

In 2.49 there are three buttons called “Sky, Premul, and Key”. Sky is the default, but you probably want Key. Press F10, then click the “Key” button and render a test.

3

Hmmm… key gives me the right colours, but with 2-tone alpha.

Premul gives me the right alpha, but still makes transparent areas darker.

I need some way to composite the output - with the colour from Key and the alpha from Premul.